This application claims priority to an application entitled xe2x80x9cDevice and Method for Tracking Mobile Station""s Position in CDMA Mobile Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Dec. 1, 1998 and assigned Serial No. 98-52235, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a mobile communication system, and in particular, to a device and method for tracking a mobile station""s position.
2. Description of the Related Art
A proposed radio access standard, IS-95B, for CDMA (Code Division Multiple Access) systems provides a position tracking service for a mobile station. In accordance with a power-up function (PUF), defined by the IS-95 standard, several base stations work in conjunction and detect a mobile station""s position through measurement of a propagation delay time for a signal transmitted from the mobile station. The propagation delay time may be measured by analyzing the chip delay of a PN sequence transmitted from the mobile station over a radio link.
In the CDMA mobile communication system, the base station generates a system reference time using the global positioning system (GPS). The mobile station can then synchronize its system reference time with the base station""s system reference time. This is accomplished when, upon power-on, the mobile station receives a pilot channel and a sync channel transmitted from the base station and performs an initial sync acquisition for synchronizing its PN (Pseudo Noise) sequence to a PN sequence included in the channels.
In actuality, there is a time difference between the mobile station""s system reference time and the base station""s system reference time due to an inherent propagation delay. That is, the mobile station""s system reference time lags behind the base station""s system reference time by a time equal to the propagation delay time. The propagation delay time occurs because the base station""s PN sequence is transmitted to the mobile station over the radio link and the mobile station synchronizes its PN sequence to the base station""s PN sequence which has already undergone a propagation delay. The propagation delay happens due to the distance between the base station and the mobile station and appears as a function of time with respect to variations in the channel conditions such as the movement of the mobile station.
In the CDMA mobile communication system, the propagation delay time can be measured by analyzing the chip delay of the PN sequence received over the radio link. In general, the mobile station adjusts its system reference time (hereinafter, referred to as a mobile station""s system reference time adjust amount) by analyzing the chip delay of the PN sequence transmitted from the base station. This maintains synchronization of the PN sequence between the mobile station and the base station by compensating for the chip delay caused by the propagation delay.
Fundamentally, however, the mobile station""s PN sequence lags behind the base station""s PN sequence by a predetermined number of chips. This is because the mobile station""s PN sequence and the base station""s PN sequence become different after initial synchronization due to a propagation delay that occurred during acquisition of the initial synchronization.
The mobile station transmits a message including its PN sequence to the base station. The base station measures a propagation delay time of the message and transmits the propagation delay time information back to the mobile station. Then, the mobile station acquires a time where the propagation delay time information is received, i.e., the base station""s system reference time at the present time by using the mobile station system reference time adjust amount from the message transmission time to the propagation delay time information arrival time. The acquired base station""s system reference time is an actual base station""s system reference time before propagation delay (hereinafter, referred to as an absolute system time).
FIG. 1 shows a conventional position tracking procedure. Referring to FIG. 1, a service base station transmits a PUF message in a traffic channel control state in order to acquire a mobile station""s positional information. Upon receipt of the PUF message, the mobile station transmits a PUF pulse to the service base station while increasing the transmission power lever step by step. At the same time, the mobile station transmits the PUF pulse to adjacent base stations (not shown). Since the distance between the mobile station and the adjacent base stations is typically greater than the distance between the mobile station and the service base station, the mobile station transmits the PUF pulse to the adjacent base stations at the transmission power level which is much higher than the normal transmission power level for communication with the service base station. The service base station and the adjacent base stations measure the propagation delay time of the received PUF pulse and calculate the mobile station""s positional information. After calculating the mobile station""s positional information, the service base station transmits a PUF completion message to the mobile station. The PUF completion message includes the latitude, longitude and measure time information. The mobile station then stops transmission of the PUF pulse.
In the conventional position tracking method, it is possible to track the mobile station""s position only by request of the base station in an active station (i.e., the traffic channel control state). That is, position tracking for the mobile station can be performed only during a call, therefore, it is not possible to provide a continuous position tracking service.
In addition, when a PUF message is sent from the service base station, the mobile station transmits the PUF pulse to the non-service adjacent base stations at a transmission power level which is much higher than the normal transmission power level for communication with the service base station. This causes an increase in interference with the reverse link. This will not only reduce the traffic capacity of the reverse link but it will also increase the power consumption of the mobile station.
As described above, the conventional method for tracking a mobile station""s position is disadvantageous in that the PUF pulse transmitted from the mobile station to the adjacent base stations causes an increase in interference with the reverse link. Measuring the phase differences, dependent on propagation delay, of tone signals transmitted from the adjacent base stations enables the mobile station to calculate its positional information. This does not require the transmission of the PUF pulse. In this manner, the mobile station should know beforehand an absolute system time that corresponds to each base station. This absolute system time is a reference time that is used for the measurement of the propagation delay of the respective tone signals.
It is, therefore, an object of the present invention to provide a device and method for calculating positional information of a mobile station on demand.
It is another object of the present invention to provide a device and method for calculating positional information of a mobile station, using the phase delay of tone signals transmitted from adjacent base stations and positional information of the adjacent base stations.
It is a further another object of the present invention to provide a device and method for calculating positional information of a mobile station using phase differences associated with tone signals transmitted at different time points from the respective adjacent base stations on the basis of an absolute system time.
To achieve the above and other objects of the present invention, there is provided a device for tracking a mobile station""s position in a CDMA (Code Division Multiple Access) mobile communication system. The device comprises a receiver for receiving single tone signals and positional information transmitted from at least three base stations; a tone phase detector for measuring phase differences, dependent on propagation delay, of tone signals transmitted from a plurality of base stations on the basis of the absolute system time; and a mobile station""s position calculator for calculating positional information of the mobile station using the measured phase differences of the tone signals and positional information of the base stations.